The present invention relates to a cardiac pacemaker electrode for transvenous application, the electrode being of the type including an insertion lead and a wire spring arrangement in electrical contact therewith for fastening in a heart wall. During the process of inserting such an electrode, the wire spring arrangement is held in a hollow cylinder and is connected with a piston which is displaceably mounted so that the pressure exerted on the piston by a guide wire conducted within the insertion lead causes the wire spring arrangement to exit from the hollow cylinder and to take on its position to engage in the cardiac tissue.
Such a cardiac pacemaker electrode is disclosed in U.S. Pat. No. 3,814,104. In that arrangement, there exists the drawback that, in the implanted state, reliable contact is not assured with certainty between the helical insertion lead and the wire spring arrangement which produces the actual stimulation in the cardiac tissue and is connected with the piston. Interruptions in the contact which interfere with the operation of the cardiac pacemaker, particularly in connection with the monitoring of the signals derived from the heart, and higher transfer resistances within the electrode arrangement reduce the voltage of the transmitted pulses so that their effect is reduced and proper cardiac stimulation is no longer assured.
In some types of pacemakers, such an increased transfer resistance can be compensated by an increase in the output pulse amplitudes. But the result of this is increased energy consumption and thus shorter life of the energy sources of the pacemaker, so that premature reimplantation may become necessary.
In the prior art electrode the ends of the wires forming the wire spring arrangement, when in the extended and thus spread open state, contact the electrically conductive hollow cylinder as soon as they are relaxed. But since the wire ends penetrate into the cardiac wall tissue to fix the electrode, they are guided essentially in an axial direction during penetration into the tissue. As a result, and considering the holding forces acting on them during the movement of the heart itself, they are only temporarily in secure mechanical, and thus electrical, contact with the front edge of the electrically conductive hollow cylinder. Since the conductive connection through the displaceable piston may also temporarily change its resistance value, there exists the danger of the creation of artifacts on the transmission path between the locus of stimulation and the pacemaker circuitry.